Natural gamma-ray logging currently is used as a qualitative indicator of uranium mineralization in an earth formation penetrated by a borehole, i.e., a high gamma-ray count rate on a natural gamma-ray log suggests a mineralized region. The natural gamma-ray logs obtained by conventional practice cannot be used as a reliable quantitative measure of uranium ore grade. This is due primarily to conditions of disequilibrium which exist between the parent uranium-238 and the daughter elements, especially bismuth-214, which emit the bulk of gamma rays contributing to the natural gamma-ray log. Disequilibrium has been found to exist when the radioactive daughter elements, through various processes such as leaching, become separated from the parent uranium. If sufficient time has not elapsed between the separation of the uranium parent and the radioactive daughter elements, a relatively low natural gamma-ray activity may be present at the actual ore body. On the other hand, one can find high natural radioactivity coming from the separated daughters with little or no uranium present.
In addition to the above, other elements, particularly potassium and thorium, emit natural gamma radiation. This radiation, when detected and recorded, reduces the effectiveness of a natural gamma-ray log as a quantitative measure of uranium ore grade.
Current exploration practice is carried out by drilling exploration holes extensively on a widely separated pattern and very extensively on a closer pattern after a good show of mineralization is found by natural gamma-ray logging. Core holes are drilled and the cores extensively assayed chemically to quantitatively evaluate the ore deposit. This practice, however, is very expensive. For example, the costs of coring a hole and chemically assaying the cores are seven to ten times the cost of an exploration hole. Moreover, the presently used technology misses many ore bodies because all natural gamma-ray anomalies suggestive of ore grade mineralization cannot be confirmed because of the prohibitive costs of coring and chemical assaying.